1. Field of the Invention
The present invention relates to the correction of beam current fluctuations in electron beam inspection tools and other similar apparatus.
2. Description of the Background Art
In electron beam inspection systems, defects are often detected by comparing the signals from corresponding image pixels in a chip (die) being tested and a reference standard. The reference standard may be an electronic database (for die-to-database inspection) or a reference die (for die-to-die inspection). A defect is typically detected when the signals between the die and reference differ by more than a threshold amount.
Beam current fluctuations may cause errors in such defect detection. These beam current fluctuations may be caused, for example, by emission noise from cold field or Schottky electron emission sources, or from other causes.
A conventional technique for correcting beam current fluctuations uses a circuit connected to the beam-limiting aperture to measure an electrical current from the aperture. This electrical current is due to the electrons being absorbed by the aperture. From the current measured, a beam current may be inferred. Changes in the current measured are used to infer changes in the beam current.